1. Field of the Invention
This invention relates to a pneumatic tire comprising a belt and a belt reinforcement embedded with reinforcing elements extending in a circumferential direction.
2. Description of Related Art
Heretofore, there has been known a pneumatic tire comprising a belt reinforcement for reinforcing a belt as disclosed, for example, in JP-A-2-208101. Such a tire comprises a carcass toroidally extending between a pair of bead portions, a belt arranged on an outside of the carcass in a radial direction and comprised of at least two belt layers embedding many reinforcing cords inclined with respect to an equatorial plane of the tire, the cords of which layers being crossed with each other, a belt reinforcement arranged on an inside of the belt in the radial direction and having a width narrower than that of the belt and comprised of at least one belt reinforcing layer embedded with reinforcing elements extending in a circumferential direction, and a tread rubber arranged on outsides of the belt and the belt reinforcement in the radial direction.
However, if it is intended to be low aspect ratio trend of such a pneumatic tire in accordance with the higher speed requirement and lowering of floor in recent vehicles, since the restraint by the belt reinforcement is insufficient, a tread portion, particularly a shoulder portion largely grows in the radial direction in the inflation under an internal pressure, and hence it is apt to generate separation failure between the belt layers at their widthwise outer ends by such a size growth of the tread portion.
For this end, it is recently examined to enhance the restraint to the tread portion by making the width of the belt reinforcement wider than the width of the belt to thereby uniformize the size growth of the tread portion and control the separation failure between the belt layers. However, when the width of the belt reinforcement is made wider than the width of the belt, it has been confirmed that separation failure is generated at the belt reinforcement, particularly in a widthwise outer end of a widest-width belt reinforcing layer.
The inventor has made various studies on such a separation failure at the end of the belt reinforcement and obtained the following knowledge. Firstly, the tread portion is subjected to compression deformation in the radial direction in a ground contact region of the tread portion, particularly a portion just under a load or in neighborhood thereof, and such a compression deformation becomes fairly large in an end part of the tread portion located in a position corresponding to a widthwise outer end of the belt reinforcement, particularly the widest-width belt reinforcing layer in case of a low-section profile pneumatic tire. Secondly, when the belt and the belt reinforcement are deformed so as to narrow their widths by the above compression deformation, a coating rubber surrounding the reinforcing element located at the widthwise outermost end of the widest-width belt reinforcing layer is subjected to deformation in the widthwise direction, and if a JIS hardness of a lateral rubber located adjacent to such a coating rubber is smaller than a JIS hardness of the coating rubber, the lateral rubber hardly controls the deformation of the coating rubber in the widthwise direction and hence a large strain is caused in the coating rubber to facilitate the occurrence of separation failure.
The invention is based on the above knowledge and lies in a pneumatic tire comprising a carcass toroidally extending between a pair of bead portions, a belt arranged on an outside of the carcass in a radial direction and comprised of at least two belt layers containing many reinforcing cords inclined with respect to an equatorial plane of the tire, the cords of which layers being crossed with each other, a belt reinforcement arranged on an inside of the belt in the radial direction and having a width narrower than that of the belt and comprised of at least one belt reinforcing layer embedded with reinforcing elements extending in a circumferential direction, and a tread rubber arranged on outsides of the belt and the belt reinforcement in the radial direction, in which a widthwise outer end of a widest-width belt reinforcing layer is arranged outward from a widthwise outer end of a widest-width belt layer among the belt layers, and a restraining rubber having a width of not less than 4 mm and a JIS hardness not less than a JIS hardness of a coating rubber for the widest-width belt reinforcing layer is arranged outward from the widthwise outer end of the widest-width belt reinforcing layer.
When the restraining rubber having a JIS hardness not less than that of the coating rubber for the widest-width belt reinforcing layer is arranged outward from the widthwise outer end of the widest-width belt reinforcing layer as mentioned above, the hardly deformable restraining rubber restrains the coating rubber surrounding the reinforcing element located at the widthwise outermost end of the widest-width belt reinforcing layer to control the deformation of the coating rubber in the widthwise direction. Thus, strain in the coating rubber is decreased to control the separation failure. Moreover, when the width of the restraining rubber is less than 4 mm, the above restraining force becomes too weak and hence the effect of controlling the separation failure can not be developed.
In a preferable embodiment of the invention, the JIS hardness of the restraining rubber is 65-85 degrees. Thus, the separation failure at the widthwise outer end of the widest-width belt reinforcing layer can effectively be controlled while preventing the occurrence of separation failure in the carcass.
In another preferable embodiment of the invention, a gauge of the restraining rubber at a boundary between the widthwise outer end of the widest-width belt reinforcing layer and the restraining rubber is made not less than a thickness at the widthwise outer end of the belt reinforcement. Thus, the occurrence of separation failure can effectively be controlled in any position at the widthwise outer end of the widest-width belt reinforcing layer.
In the other preferable embodiment of the invention, the restraining rubber is integrally united with the coating rubber for the belt reinforcing layer. Thus, the restraining rubber is arranged together with the arrangement of the belt reinforcing layer, so that the working operation becomes easy.
In a further preferable embodiment of the invention, the restraining rubber is extended inward in the widthwise direction so as to cover the widthwise outer end part of the belt reinforcement from the outside thereof in the radial direction. Thus, strain is largely decreased at the widthwise outer end of the widest-width belt reinforcing layer, whereby the separation failure can be controlled more effectively.
In a still further preferable embodiment of the invention, when the JIS hardness of the restraining rubber is not less than 65 degrees but not more than 80 degrees, the restraining rubber is passed over an outside of the widthwise outer end part of the belt reinforcement in the radial direction and extended inward in the widthwise direction up to a zone between the belt reinforcement and a widthwise outer end part of a widest-width belt layer. Thus, separation failure at the widthwise outer end of the widest-width belt reinforcing layer and separation failure between the belt reinforcement and the belt can simultaneously controlled.
In a yet further preferable embodiment of the invention, the restraining rubber is extended inward in the widthwise direction so as to envelop the widthwise outer end part of the belt reinforcement from its outside and inside in the radial direction. Thus, strain at the widthwise outer end of the widest-width belt reinforcing layer is surely decreased, whereby the separation failure can strongly be controlled.
In another preferable embodiment of the invention, the restrained rubber is comprised of two or more radially laminated rubber layers having different JIS hardnesses, and a JIS hardness of a rubber layer located near to the carcass among the rubber layers becomes smaller than that of the remaining rubber layer. Thus, strain in a coating rubber for the carcass adjacent to the restraining rubber can be decreased.
In the other preferable embodiment of the invention, when the JIS hardness of the restraining rubber exceeds 85 degrees, a rubber layer having a JIS hardness smaller than that of a coating rubber for the carcass is interposed between the carcass and the restraining rubber. Thus, the occurrence of separation failure in the coating rubber for the carcass can be prevented.